(1) Field of the Invention
This invention relates to a method for producing high strength sintered silicon carbide, and more particularly to a method for producing high strength sintered silicon carbide by a hot isostatic pressing treatment (hereinafter referred to as "HIP" for brevity) in a nitrogen gas atmosphere of preliminarily sintered silicon carbide.
(2) Description of the Prior Art
Recently great concern is directed to non-oxide ceramics like silicon nitride (Si.sub.3 N.sub.4) and silicon carbide (SiC) which are superior to the conventional oxide ceramics as represented by alumina (Al.sub.2 O.sub.3) in high temperature strength and resistance, for their possibilities of providing good heat resistant structural materials in the future for gas turbines, Diesel engines and heat exchangers which are operated at high temperatures.
However, in spite of the above-mentioned excellent physical properties, the non-oxide ceramics like Si.sub.3 N.sub.4 and SiC have a number of problems to be solved before actual application, arousing instensive studies in various research organizations in and outside this country. One of the most important problems which are involved in this regard is the establishment of the technology of producing parts of high strength and complicated shapes. Although vigorous research activities are directed particularly to Si.sub.3 N.sub.4 these days, it is found difficult to sinter Si.sub.3 N.sub.4 alone due to its strong covalent bonding property. Therefore attempts have been made to densify it with a small amount of sintering aid by hot press sintering, ordinary sintering or reaction sintering. However, these attempts respectively have own merits and demerits and they have not yet arrived at the establishment of any satisfactory method.
On the other hand, in step with the recent development of the molding by the hot isostatic pressing (HIP), it has been considered to apply the HIP molding techniques to the Si.sub.3 N.sub.4 sintering. For example, Larker et al reported in HIGH PRESSURE SCIENCE AND TECHNOLOGY, vol. 12 (1972) that, even when the additive amount of yttrium oxide (Y.sub.2 O.sub.3) as a sintering aid was 0.5 wt %, it was possible to obtain sintered products of theoretical density by subjecting rubber-pressed Si.sub.3 N.sub.4 compacts to a HIP treatment in argon gas atmosphere after sealing them in a quartz glass container. Further, Shimada presented the results of his survey on the relationship between the densification of HIP-treated material and crystal phase of Si.sub.3 N.sub.4 in his paper for a seminar relating to "HIP technology and it use" in 1980, HIP-treating Si.sub.3 N.sub.4 powder in a manner similar to Larker et al after adding various kinds of sintering aids.
These methods, however, require to use a glass container or the like in the HIP treatment so that it still takes some time before their application for solving a number of problems, e.g., means for producing containers of complicated shapes, a method for uniformly filling Si.sub.3 N.sub.4 in the container, a method for preventing reaction between Si.sub.3 N.sub.4 and the container, and a method for detaching the Si.sub.3 N.sub.4 from the container.
Under these circumstances, the applicants proposed in their prior application a method for producing high strength sintered Si.sub.3 N.sub.4 with a relative density higher than 98%, by compacting Si.sub.3 N.sub.4 powder into a predetermined shape, preliminarily sintering the resulting green compact to a relative density of 92% and then subjecting the preliminarily sintered compact to a HIP treatment at a temperature higher than 1500.degree. C. and under direct action of a hot high-pressure gas with a partial nitrogen gas pressure higher than 500 atms.
In contrast to the energetic activities for the development of a HIP process which is applicable to Si.sub.3 N.sub.4, there have thus far been made almost no proposals with regard to the application of HIP to SiC which belongs to the same non-oxide ceramics. However, it should be recognized that the application of the HIP treatment to SiC has the same importance as it has to Si.sub.3 N.sub.4 in consideration of potential utility of SiC.